This invention was made under contact with or supported by the Electric Power Research Institute, Inc. of Palo Alto, California.
1. Field of Invention
This invention relates to the fabrication of solid state devices and relates particularly to self-protection of thyristor devices and the like.
A thyristor is a solid state device having alternate layers of P type and N type semiconductor materials. A thyristor is typically a disc of four alternating layers of N and P type silicon, the layers and junctions between them being formed by precision gaseous diffusion, substrate fusion and/or alloying techniques. A thyristor has generally three electrodes, referred to as the cathode, the anode, and the gate, the gate being the control electrode for the device. During normal operation, the thyristor is turned on by at least momentary application of a forward bias gate-to-cathode voltage. The device remains on until the anode-to-cathode voltage is reduced to a value below that required to sustain regeneration, or forward current.
The thyristor may also be turned on without a voltage applied to the gate if the anode-to-cathode voltage exceeds a value inherent in the device design. This phenomenon is known as voltage breakover turn-on. The main emitter area in the cathode of a thyristor is prone to failure during breakover turn-on initiated by such excess device voltage. The location of the turn-on point within the device is not usually subject to control. As a result, the turn-on point may often occur within the cathode emitter in a manner causing permanent device failure.
The device turn-on criterion, which is based on the current gain of the transistor formed by the anode layer, the anode base layer and the cathode base layer of the thyristor device, is approximated to the first order as the product of the anode base transport factor .alpha..sub.T and the avalanche multiplication factor M. (Strictly speaking, the turn-on criterion is the product of the current gain factor, .alpha..sub.0, and the multiplication factor, M. However, because current gain, .alpha..sub.0, is the product of the anode base transport factor, .alpha..sub.T, and the emitter efficiency, .gamma..sub.E, of the anode emitter, which is close to unity and does not vary strongly with voltage, to the first order .alpha..sub.0 .apprxeq..alpha..sub.T. Therefore, the subscript may be ignored and the terms current gain and base transport factor may be used interchangeably.) Both the base transport factor and the avalanche multiplication factor are voltage sensitive parameters. An excess forward voltage will cause the product .alpha..M to prematurely exceed unity within selected regions of the thyristor, resulting in a local current gain approaching infinity. Device turn-on in this manner often causes device failure.
2. Description of the Prior Art
In the past, there have been two basic methods for protecting against voltage breakover turn-on failure. In the first method, external circuitry is connected between the anode and the gate which has a breakover voltage below that of the internal emitter to be protected. As voltage between the anode and cathode approaches the breakover level, the resulting avalanche current in the external circuitry becomes the gate current of the thyristor, thereby firing the thyristor normally. One of the major shortcomings of this type of breakover protection is the need for additional external circuit components, with the resultant increased expense and system size.
A second method involves the use of internal auxiliary fabrication techniques, wherein the silicon wafer from which the N type base region is fabricated is prepared so that the highest donor concentration is located precisely below the area for the gate contact. This method is described in an article by Peter Voss, Solid State Electronics, Volume 27, page 265 (1974). In the Voss method, the dependence of the avalanche breakdown characteristic on donor concentration assures that the doped region is the first region in which breakdown can occur, thereby protecting against voltage breakover turn-on failure in any other region of the thyristor.
Thyristors and many other semiconductor devices are fabricated from a wafer of silicon, initially of high purity, which is characterized by a long charge carrier lifetime. In the course of fabrication it is common practice to uniformly irradiate the wafer or to introduce a lifetime reducing impurity uniformly into the wafer surface to modify the characteristic of the silicon wafer. This technique can be adapted to advantage as hereinafter described.